1. Field of the Invention
This invention is related to light pipes that are used for illumination. More particularly, this invention is directed to an illuminating light pipe that is used to illuminate an elongate area along which the light pipe extends.
2. Description of the Prior Art
Light pipes or light guides can be employed for illumination as well as for high speed signal communication. When used for illumination, light pipes can either be used to transmit light from a source to a remote area to be illuminated, or the light pipes can be used to illuminate an area along which the light pipe extends. If a light pipe has a smooth outer surface with few imperfections, the index of refraction between the light pipe and air will be sufficient to reflect most of the light, propagated through the light pipe or fiber, so most of the light will be transmitted between a light source at one end and the other end of the fiber optic cable or pipe. However, to illuminate the elongate area along the light pipe of fiber, light must be emitted laterally, and to uniformly illuminate this elongated strip or path light should be emitted at comparable levels along the light pipe. Of course, one approach is to use a multi-fiber cable with fibers of different lengths arrayed to emit light from the ends of the individual fibers at points spaced along the length of the multi-fiber cable. However, deployment of cables or fibers at discrete locations would require a relatively complicated and costly assembly.
Optical fiber lighting apparatus can employ several techniques for scattering light propagating within the light fiber of light pipe into lateral modes of illumination to illuminate the area surrounding the light pipe. One approach is to employ scattering centers within the core to scatter propagating light outwards so that light is emitted laterally in the vicinity of the scattering centers. Another approach is to roughen portions of the core layer at the interface with the clad layer of the optical fiber to scatter light impinging on the core-clad interface. By roughing the surface, the index of refraction is altered so that more light can be laterally emitted from the light pipe or fiber. Although the core dimensions of the fiber or pipe remain constant for these two approaches, the light pipe is not uniform along the length of the pipe. Either discrete areas along the axis or length of the fiber or pipe must be roughened, or the scattering centers must be added at discrete locations. Therefore, the pipe or fiber cannot be fabricated on a continuous basis, or if a continuous fiber is used, it must be subjected to secondary operations to roughen the surface.
If the surface of a cable is roughened along the entire length of the fiber, either at the core-clad interface, or along the exterior of a fiber that does not include a clad layer, light will not be emitted uniformly along the length of the fiber or pipe. A large portion of lateral light emission will occur adjacent to the light source and the lateral light emission will be significantly less near the remote end of the light pipe. A light pipe that is continuously roughened in this manner is not suitable for use in providing substantially uniform illumination along an elongate surface or area, such as along step or the edge of a structure.
Another approach that has been employed is to position reflectors along one side of a light pipe or fiber optic cable so that light emitted from one side will be reflected back onto an area on the opposite side of the light pipe. One approach employs multiple reflectors with progressively varying indices of reflection located along the length of the cable. This approach relies on the variation of the index of reflection of the separate reflectors to overcome the variation in the illumination along the length of the cable. Multiple reflectors or mirrors are necessary for this approach so that it would not constitute a low cost solution to the problem.
One other approach varies the geometry of the cable along its length, but this nonuniform cable would be more costly than a light pipe having a constant cross sectional area.
Each of these approaches is unsatisfactory when used to provide uniform illumination along a surface or edge of a structure to highlight this structure for safety or other reasons. For example, it is desirable to use a light pipe to illuminate a step up surface on a motor vehicle to increase the visibility and therefore the safety of such a feature. For both practical and aesthetic reasons, the illumination should be uniform along the length of such an elongate structure, or at least the feature should be comparably illuminated along its length. Since the light pipe is intended to illuminate the feature, a slight light intensity variation that would not be noticeable to an observer or user would still be acceptable. One attempt to provide a low cost light pipe of this type was to paint one side of a light pipe that would be mounted in a slot or channel on the vehicle. The paint would provide an opaque surface on one side that would hopefully reflect light back through the light pipe onto the exterior of the structure on which this light pipe was mounted. However, it was found that most of the light was emitted adjacent the light source and the area at remote end of the light pipe was inadequately illuminated.